Researchers Use New Method to Observe Spin Behavior of Electrons in Quantum Material

A new technique that precisely measures the mysterious behavior and magnetic properties of electrons that flow across the surface of quantum materials could lead to the next generation of electronics.

Silicon-based semiconductors, found at the center of electronic devices, depend on the controlled electrical current which is responsible for powering electronics. They can access only the electrons’ charge for energy; however, electrons do more than just carry a charge. These semiconductors also have intrinsic angular momentum called spin, which is an aspect of quantum materials that, while elusive, can be controlled to improve devices.

A group of Researchers, headed by An-Ping Li from the Department of Energy’s Oak Ridge National Laboratory, has developed a new microscopy method to detect the spin of electrons in topological insulators, an innovative type of quantum material that could be employed in applications such as quantum computing and spintronics.

The spin current, namely the total angular momentum of moving electrons, is a behavior in topological insulators that could not be accounted for until a spin-sensitive method was developed.

An-Ping Li, The Department of Energy’s Oak Ridge National Laboratory

Electronic devices continue to grow rapidly and need more power compressed into smaller components. This causes the need for less expensive, energy-efficient substitutes to charge-based electronics. A topological insulator carries electrical current along its surface, and when deeper inside the bulk material, it serves as an insulator. Electrons flowing across the surface of the material display uniform spin directions, unlike in a semiconductor where electrons spin in different directions.

Charge-based devices are less energy efficient than spin-based ones. For spins to be useful, we need to control both their flow and orientation.

An-Ping Li, The Department of Energy’s Oak Ridge National Laboratory

In order to detect and understand the quirky particle behavior, the group required a technique that is sensitive to the spin of moving electrons. This new microscopy technique was tested on a single-crystal Bi₂Te₂Se, a material containing selenium, tellurium and bismuth. It measured how much voltage was generated along the surface of the material as the flow of electrons moved between particular points while sensing the voltage for each spin of the electron.

The new technique uses a four-probe scanning tunneling microscope — a device that can identify the atomic activity of a material with four movable probing tips — by including a component in order to observe the spin behavior of electrons on the surface of the material. This method not only contains spin sensitivity measurements, but also compresses the current to a small area on the surface, which assists in keeping electrons from escaping below the surface, offering high-resolution results.

“We successfully detected a voltage generated by the electron’s spin current,” stated Li, who co-authored a paper published by Physical Review Letters that explains the method.

This work provides clear evidence of the spin current in topological insulators and opens a new avenue to study other quantum materials that could ultimately be applied in next-generation electronic devices.

An-Ping Li, The Department of Energy’s Oak Ridge National Laboratory

Other Co-authors of the research titled, “Detection of the Spin-Chemical Potential in Topological Insulators Using Spin-Polarized Four-Probe STM,” include Arthur Baddorf, Wonhee Ko, Giang Nguyen, and Saban Hus from ORNL; Yong Chen of Purdue University; and X.-G. Zhang of the University of Florida.

This research was performed at the Center for Nanophase Materials Sciences, a DOE Office of Science User Facility. ORNL’s Laboratory Directed Research and Development program funded the development of the new microscopy technique.